A CMOS-process-based high performance lateral power MOSFET, in particular, a trench lateral power MOSFET (hereinafter referred to as a TLPM), has been recently developed. The TLPM has reduced on-resistance and higher integration when integrated into a power IC, as compared with a conventional planer lateral power MOSFET. See for example JP-A-2002-184980 and JP-A-2002-270831.
In this regard, a Japanese patent application, JP-A-2002-353447, by the present inventors, discloses a planar layout of a TLPM on a mesh pattern for which a trench region is formed like a mesh with each of the regions that are not subjected to trench etching left as an island, i.e., trench surrounding the non-trench regions. FIG. 126 illustrates a basic pattern of the mesh-shaped trench pattern disclosed in the above identified patent application. In FIG. 126, a hatched region is a trench etching region 1 surrounding the non-trench or non-etched regions 2. Moreover, reference numerals 3 and 4 denote a drain contact and a drain electrode, respectively, and reference numerals 5 and 6 denote a source contact and a source electrode, respectively. An active region comprises the trench region Wt extending between the non-trench regions 2 with the source electrode 6 formed therein. A gate insulator film and gate polysilicon are formed in the trench region, and source polysilicon and the like are further formed, in the active region. At the bottom of the trench, a source region, a base region, a body region, etc., are formed. In the non-trench region 2, a drain region, an expanded drain region, etc., are formed.
Incidentally, it is generally desirable for a MOSFET to have a lower on-resistance per unit area. An important parameter for determining the on-resistance per unit area is called a channel width per unit area (hereinafter referred to as a channel density). With the channel width taken as Wch and an element area expressed as A, the value of the channel density P is given by the following Equation (1). For a high integration of a transistor to enhance a driving capability of a current per unit area, it is better to have a larger channel density P:P=Wch/A.  (1)
For convenience of explanation, in FIG. 126, the non-trench region 2 is designated as a rectangle STUV, where, a length of a side extending parallel to the longitudinal direction of the drain electrode 4 or the source electrode 6 of the rectangle STUV (hereinafter referred to as the longitudinal direction), and a length of a side extending perpendicular to the longitudinal direction (hereinafter referred to as the lateral direction) are designated as St and Lt, respectively. Moreover, the distance between a pair of adjacent non-trench regions 2 in the longitudinal direction is designated as Wg, and the distance between the non-trench regions 2 adjacent to each other in the lateral direction is designated as Wt.
Furthermore, a rectangle, formed by two adjacent center lines EF and HG of trenches extending longitudinally and two adjacent center lines EH and FG of trenches extending laterally intersecting with each other, is designated as EFGH. The area A of the rectangle EFGH is expressed by the following Equation (2):
                                                        A              =                            ⁢                                                (                                                            Wt                      /                      2                                        +                    Lt                    +                                          Wt                      /                      2                                                        )                                ×                                  (                                                            Wg                      /                      2                                        +                    St                    +                                          Wg                      /                      2                                                        )                                                                                                        =                            ⁢                                                (                                      Wt                    +                    Lt                                    )                                ×                                                      (                                          Wg                      +                      St                                        )                                    .                                                                                        (        2        )            
In the mash pattern shown in FIG. 126, a channel is formed around the non-trench region 2. Therefore, the channel width Wch in each rectangle EFGH is expressed by the following Equation (3):Wch=2(Lt+St)  (3)Thus, from the previous Equation (1), the above Equation (2) and the Equation (3), the channel density P is given by the following Equation (4):
                                                        P              =                            ⁢                              2                ⁢                                                      (                                          Lt                      +                      St                                        )                                    /                  A                                                                                                        =                            ⁢                              2                ⁢                                                      (                                          Lt                      +                      St                                        )                                    /                                                            {                                                                        (                                                      Wt                            +                            Lt                                                    )                                                ×                                                  (                                                      Wg                            +                            St                                                    )                                                                    }                                        .                                                                                                          (        4        )            
The above-explained mesh-shaped trench pattern provides the value of the channel density P becoming larger than that provided by a conventional stripe-like trench pattern. Namely, in a stripe pattern shown in FIG. 127, no trench is formed in the lateral direction. Nevertheless, by setting a rectangle EFGH in the same way as that in FIG. 126, the channel width Wch in each rectangle EFGH can be expressed by the following Equation (5).Wch=2(Wg+St)  (5)Hence, the channel density P is given by the following Equation (6):
                                                        P              =                            ⁢                              2                ⁢                                                      (                                          Wg                      +                      St                                        )                                    /                                      {                                                                  (                                                  Wt                          +                          Lt                                                )                                            ×                                              (                                                  Wg                          +                          St                                                )                                                              }                                                                                                                          =                            ⁢                              2                /                                                      (                                          Wt                      +                      Lt                                        )                                    .                                                                                        (        6        )            
Here, as is apparent from FIG. 126 and FIG. 127, the value of (Lt+St) becomes larger than the value of (Wg+St). Thus, from the comparison of the above Equation (6) and the previous Equation (4), it is understood that the mesh pattern provides a channel density P that is than that of the stripe pattern. Therefore, in the TLPM with the mesh pattern, miniaturization can increase the channel density P more than in the TLPM with the stripe pattern to make it possible to reduce on-resistance per unit area.
However, miniaturization of the TLPM with the mesh pattern necessitates minimizing the dimensions of the Lt and the St, that is, minimizing the non-trench region 2. This miniaturizes the drain contact 3 because the mesh pattern shown in FIG. 126 provides a layout in which the drain contact 3 is formed within the non-trench region 2. This increases the contact resistance, making it impossible to reduce the on-resistance as much as an amount of increase in the channel density P. Moreover, this also can cause failure in continuity due to reduced contact hole openings.
In addition, the source region is presented only at the bottom of the trench in the active region without being provided in a region (a gate region) for leading out the gate polysilicon in the trench etching region 1 onto the substrate surface. This causes current to pass around insufficiently to the trench side in the gate region. Therefore, even though the channel density P is increased by the mesh-shaped trench pattern, reduced on-resistance due to the increase in the channel density P is not sufficiently realized. These problems similarly arise in all of TLPMs each being of a type with drain polysilicon formed in the trench etching region 1 and with a drain region formed at the bottom of the trench.
Accordingly, there remains a need for a semiconductor device, such as a TLPM, in which the lowered on-resistance can be realized. The present invention addresses this need.